The present invention relates to a substrate on which can be mounted electrical subsystems such as integrated circuits, multi-layered printed circuits and the like.
The use of a ceramic material as the substrate for such devices has encountered a number of technical difficulties. Where a multi-layered printed circuit is formed on the ceramic substrate, the shrinkage coefficient of the paste-like materials forming printed circuits is so unlike that of the ceramic substrate that the printed circuits crack, warp or form other defects during sintering. Unexamined Japanese Patent Publication 59-101896 discloses a method of manufacturing a multi-layered device to avoid such problems by manipulating the powder sizes of the material forming the substrate with respect to the powder size of the material forming the conductors. While the different particle sizes could be matched to eliminate or reduce the different shrinkage rates upon sintering, control of the particle size for the powder components was difficult. In addition, because the sintering temperature of the ceramic substrate was so high, the conductive metal layer had to be a metal having high thermal resistance such as tungsten or molybdenum. Such metals have high electrical resistance and are unsuitable for high speed signal transmission.
The use of a ceramic substrate in connection with an integrated circuit (IC) has similar problems. A conventional IC package is set out in FIG. 3. Such a device comprises a ceramic substrate 34 on which is mounted a semiconductor element 33 by means of an intermediate joint layer 32 which is normally a metallized or plated layer. A cap 35 protects the semiconductor element 33 and a glass layer 36 seals the substrate and the cap in an air-tight relationship. A terminal member 37 electrically connects the semiconductor element 33 by means of a bonding wire that is led through the glass layer 36.
Alumina has conventionally been used for the ceramic substrate 34 because of its mechanical strength and electrical insulating properties. Use of alumina is not without some technical drawbacks. Alumina has a high dielectric constant, such that it causes delays in signal propagation and its sintering temperature is so high that high melting point metals, such as tungsten and molybdenum or the like, must be used as the signal conductive member and such materials have a high electrical resistance.
Crystalline glass or glass-ceramic materials have also been used in such applications. While the dielectric constant is low and the sintering temperature is low enough to allow use of high conductivity metals such as copper for the signal conductive member, such materials have the disadvantge that the low thermal conductivity adversely affects the heat discharge characteristics of the device.
Ceramic substrates are also used in connection with LSI devices. Conventionally, ceramic materials such as alumina are molded into a sheet-like form, a wiring pattern is formed on the ceramic sheet and a multi-layer structure formed by stacking several of such sheets. The resulting multi-layer structure is sintered. Where such devices are used in information processing apparatus, there has recently been a strong incentive to miniaturize such devices and to improve the packing density by mounting a number of LSI elements on one ceramic substrate. Where the degree of LSI integration is low, the formation of the wiring pattern on only one surface of the substrate is sufficient, therefore, metals having a low electrical resistance such as gold, silver or copper may be used as the wiring material. On the other hand, when the degree of integration is relatively high and there are a relatively large number of LSI elements on the substrate, it is necessary to form the wiring pattern on each of the ceramic sheets constituting the substrate. The ceramic sheets are stacked one on top of the other to form a three-dimensional wiring array that simplifies the wiring between separate substrates. Where conventional ceramics have been used for the LSI substrate, however, it has been necessary to sinter the substrate at high temperatures, for example, in the range of from 1350.degree. to 1600.degree. C. when alumina is used as the ceramic substrate. At such sintering temperatures low melting point metals such as gold, silver, copper, or the like, cannot be used because such a metal will separate due to surface tension of the molten metal. Accordingly, it has been necessary to form the electrical conductors on the substrate by using a high melting point metal such as tungsten, molybdenum, or the like. Such materials, however, have a high electrical resistance in relation to gold, silver, copper, or the like. The electrical resistance of such metals as tungsten, molybdenum, or the like, is about three times as large as gold, silver or copper in resistivity and about five to ten times as large as the latter when the high resistance materials are sintered. As a result, the electrical resistance of the substrate is increased as the number of layers forming the LSI substrate are increased. This causes delays in signal transmission speed and results in a serious problem in high speed information processing apparatus.
In order to solve the above-mentioned problems, Unexamined Japanese Patent Publication No. 57-6257 discloses a means for enabling the use of low temperature sintering of substrates by using glass as the material for the substrate. Gold, silver, copper, or the like, are used for the electrical conductors and are located in paths formed within the substrate. To form such a device, however, it is necessary to perform the sintering operation in a vacuum in order to prevent the formation of voids within the electrical conductors being formed inside the substrate.
Further, it is necessary to provide circuits on the surface of a multi-layer substrate that connect the LSI chips mounted on such a surface to the electrical conductor paths formed inside the substrate. Such circuits are formed of electrically conductive layers applied to the device by vacuum evaporation, sputtering, screenprinting, or the like. As a result, a high degree of surface smoothness (microscopic evenness) and a high degree of flatness (macroscopic evenness) is required for the surface of the substrate such that the conductive layer can be accurately formed. This problem is not addressed in the Unexamined Japanese Patent Publication 57-6257.
As a result of the shortcomings in prior art devices, the present invention has as its principle object the formation of a substrate for a circuit or the like that can be readily formed into a stable electrically and thermally compatible device. It is a further object of the present invention to form such a device at relatively low temperatures. It is an additional object of the invention to problems associated with differential thermal contraction upon formation of such devices. These and other objects of the invention are accomplished by the present invention as disclosed herein.